1. Technical Field of the Invention
The present invention relates to a rotary joint wiring unit for robots, which is used for providing electric wiring between two members joined by a rotary joint, and in particular, to a rotary joint wiring unit for industrial robots, which uses a flexible printed wiring board for electric connections through the rotary joint.
2. Related Art
Industrial robots are generally provided with a base and a plurality of arms sequentially joined via rotary joints. The end arm of the joined arms has a tip end that is a wrist part on which an end effector (or an actuator), such as a hand, is usually mounted.
A cable is wired in such an industrial robot to supply electric power to motors which serve as drive sources for the arms, the end effector, and the like, and to transmit/receive control signals between the motors and a robot controller. Wiring methods that can be used to wire such a cable include an internal wiring method in which a cable is wired inside the base and the arms, and an external wiring method in which a cable is wired along an outer surface of the robot.
In using either of the above two wiring methods, the wiring structure employed is required to have rotary joints that will not block relative rotation between two members (e.g., base and arm, arm and arm, arm and wrist part, etc.).
Patent documents JP-A-H06-143186 and JP-A-H10-034588 each disclose a wiring structure of a rotary joint using the internal wiring method. In the wiring structure, a cable is wired through the center of rotation of a rotary joint to absorb the relative rotation between two members with the twisting of the cable.
Another patent document JP-A-H01-153290 discloses a wiring structure of a rotary joint using the external wiring method. This wiring structure uses a flexible sheet in which a wiring pattern is formed. The flexible sheet is fixed to two relatively rotating members, bridging therebetween, in the state of being loosened into an arc form. Thus, the relative rotation between the two members is ensured to be absorbed by the arc loosened portion of the flexible sheet.
Although not related to robotic rotary joints, a patent document JP-A-2003-324835 discloses a spiral cable unit that electrically connects the steering wheel of the vehicle with the vehicle body. The spiral cable unit is provided with a casing having an outer cylinder and an inner cylinder which is disposed inside the outer cylinder so as to be rotatable relative to the outer cylinder. The casing accommodates in its interior a band of long flexible flat cable, being wound about the outer periphery of the inner cylinder. One end of the flexible flat cable is connected to the inner cylinder and the other end thereof is connected to the outer cylinder.
With the structure disclosed in JP-A-H06-143186 and JP-A-H10-034588, the relative rotation between the two members is absorbed by the twisting of the cable. Therefore, if the portion corresponding to the center of rotation of a rotary joint for wiring the cable has a small length, the rate of twisting per unit length will be high. The high rate of twisting is likely to hasten damage or disconnection of the cable. Therefore, the portion corresponding to the center of rotation of the rotary joint is required to be elongated. This means that the space required for wiring the cable has to be increased in the direction of the rotation axis of the rotary joint. The increase of such a space will increase the size of the rotary joint, which will be a disadvantage in a small-size robot, in particular, in which a number of parts are closely arranged.
In the structure disclosed in JP-A-H01-153290, the flexible sheet is flexed and extended with the relative rotation between the two members. However, since the force is concentrated in certain areas, fatigue and breakage may be accelerated especially at areas of maximum flexure.
The spiral cable unit disclosed in JP-A-2003-324835 has a structure in which the rotation of a vehicle steering wheel is absorbed by allowing the flexible flat cable to be wound about and unwound from the inner cylinder. Therefore, comparing with the structure using the twisting of a cable, the thickness can be reduced in the direction of the rotation axis of the unit. In addition, the life of the flexible flat cable (hereinafter referred to as “flexible printed wiring board”) will be lengthened because no local flexing and extending motion is imparted.
However, since the steering wheel of a vehicle is manipulated by a person, the speed of rotation is relatively slow and the wheel rotation is usually only maintained for a short time. On the other hand, when robotic arms are concerned, the speed of rotation of the arms is much higher than the speed of rotation of a steering wheel. Moreover, industrial robots are operated for a long time, which may continue as long as twenty-four hours. Thus, the spiral cable unit disclosed in JP-A-2003-324835, while suitable for absorbing the rotation of a steering wheel of a vehicle, is not configured assuming high-speed and long-duration rotation. Accordingly, it is expected that use of the flexible printed wiring board in a robotic rotary joint may problematically hasten wearing or the like. Thus, the flexible printed wiring board cannot be applied, as it is, to a robotic rotary joint.